A water heater or the like for household use is provided with an equipment that detects carbon monoxide generated when incomplete combustion occurs and notifies the risk thereof at an early stage. Such a gas component considerably affects a human body. According to the guidelines from LP gas safety committee, it is set that a carbon monoxide concentration of approximately 200 ppm (parts per million) causes headaches. Although various methods have been known as a method of detecting a gas component having a relatively higher concentration, the detection methods have been limited for detecting the gas component having a concentration of ppb (parts per billion) to ppt (parts per trillion), which corresponds to an extremely low concentration.
At a disaster site or a site at which an act of terrorism occurs or the like, it has been desired to sense the risk in advance by detecting an extremely small amount of the gas component. The gas component having an extremely low concentration is often detected by use of a large equipment in research facilities. In this case, a large sized installation type equipment, which is expensive and has large weight and volume, such as a gas chromatography or a mass spectrometer is required. Under such circumstances, it has been required to provide an apparatus that is capable of detecting the gas component having an extremely low concentration in real time, in other words, an apparatus that has a smaller weight and volume and a better portability and enables selective and higher sensitive detection of the gas component having an extremely low concentration in the order of ppt to ppb.
By the way, examples of the gas component required to be detected at an extremely low concentration include sarin, an organophosphorus compound contained in toxic agricultural chemicals, and toxic gas components such as nitrogen mustard gas and sulfur mustard gas. Besides, components of an explosive nitro compound and the like are also required to be detected at an extremely low concentration. For example, detonating explosives release an extremely small amount of volatile component even in a sealed state, and it is considered that a capability of detecting such a gas component at an extremely small concentration brings about a deterrent effect of an act of terrorism. Conventionally, it is general that a volatile component of the nitro compound or the like at a site such as an airport is detected by a trained police dog or the like. Such a gas component detection method is very effective in terms of a deterrent to an act of terrorism, but such an approach by use of an animal is high in cost and further has a difficulty in securing accuracy in a normal state.
As a detection element for the gas component having a low concentration, for example, an element has been known that has a conductive layer in which a surface of a carbon nanostructure is surface modified with an organic substance or the like that selectively reacts with or adsorbs a specific substance and measures a potential difference or the like that changes depending on the gas component that has adhered to the surface of the carbon nanostructure. In such a detection element, types of the organic substance themselves that functions as a detection probe are limited, and thus an organic substance capable of sufficiently interacting with the volatile component of the nitro compound has not been found out. In the meantime, there has been proposed a method of detecting the presence or amount of nitro compound component by providing, on a nanostructure such as a nanowire or a nanotube, a functionality component having an electron releasing group, such as amine, that bonds to the nitro compound component to form a charge transfer complex. In this method, the formation of charge transfer complex by the electron releasing group on a sensor side and the nitro compound component is likely to be affected by environmental conditions with the presence of the nitro compound component. Therefore, there is a problem that detection accuracy of the nitro compound component decreases depending on the environmental condition.